US7181903B2 - Exhaust gas purifying apparatus and method of using the same - Google Patents

Exhaust gas purifying apparatus and method of using the same Download PDF

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US7181903B2
US7181903B2 US10/845,161 US84516104A US7181903B2 US 7181903 B2 US7181903 B2 US 7181903B2 US 84516104 A US84516104 A US 84516104A US 7181903 B2 US7181903 B2 US 7181903B2
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adsorbent
exhaust gas
plasma
plasma reactor
present
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US20040231321A1 (en
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Masanori Yamato
Masaru Kakinohana
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Toyota Motor Corp
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Toyota Motor Corp
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
    • F01N3/0828—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents characterised by the absorbed or adsorbed substances
    • F01N3/0835—Hydrocarbons
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9431—Processes characterised by a specific device
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
    • F01N13/009—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series
    • F01N13/0097—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00 having two or more separate purifying devices arranged in series the purifying devices are arranged in a single housing
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
    • F01N3/0814—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents combined with catalytic converters, e.g. NOx absorption/storage reduction catalysts
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
    • F01N3/0828—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents characterised by the absorbed or adsorbed substances
    • F01N3/0842—Nitrogen oxides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2259/00—Type of treatment
    • B01D2259/80—Employing electric, magnetic, electromagnetic or wave energy, or particle radiation
    • B01D2259/818—Employing electrical discharges or the generation of a plasma
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2240/00—Combination or association of two or more different exhaust treating devices, or of at least one such device with an auxiliary device, not covered by indexing codes F01N2230/00 or F01N2250/00, one of the devices being
    • F01N2240/22—Combination or association of two or more different exhaust treating devices, or of at least one such device with an auxiliary device, not covered by indexing codes F01N2230/00 or F01N2250/00, one of the devices being a condensation chamber
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2240/00—Combination or association of two or more different exhaust treating devices, or of at least one such device with an auxiliary device, not covered by indexing codes F01N2230/00 or F01N2250/00, one of the devices being
    • F01N2240/28—Combination or association of two or more different exhaust treating devices, or of at least one such device with an auxiliary device, not covered by indexing codes F01N2230/00 or F01N2250/00, one of the devices being a plasma reactor
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2570/00—Exhaust treating apparatus eliminating, absorbing or adsorbing specific elements or compounds
    • F01N2570/22—Water or humidity

Definitions

  • the present invention relates to an apparatus for purifying exhaust gas emitted from internal combustion engine and, in particular, to an apparatus for purifying NO x (nitrogen oxide) emitted from a motor vehicle.
  • Japanese Unexamined Patent Publication NO. 2001-182525 describes an exhaust gas purifying apparatus wherein plasma is generated in a plasma reactor to enhance the adsorption of NO x by a NO x adsorbent therein, when a NO x purifying catalyst located downstream thereof is at a low temperature and does not have enough NO x purifying ability. Once the catalyst in warmed up, the trapped NO x can be desorbed and purified at the downstream NO x purifying catalyst by stopping plasma.
  • the prior art has partially overcome the problem that the NO x purifying catalyst does not have enough NO, purifying ability before it is warmed up. However, the further improvements are required because the NO x purifying ability is not always enough even if the prior art is employed.
  • the present invention provides an improved exhaust gas purifying apparatus, which overcome the above problems, and a method for using it.
  • An exhaust gas purifying apparatus comprises a plasma reactor containing at least one of a H 2 O adsorbent and a HC (hydrocarbon) adsorbent; and a NO x adsorbent located downstream of said plasma reactor.
  • Another exhaust gas purifying apparatus comprises a plasma reactor containing at least one of a H 2 O adsorbent and a HC adsorbent, and a NO x adsorbent.
  • the NO x adsorbent in the reactor may be positioned downstream of the at least one of a H 2 O adsorbent and a HC adsorbent. Further, the NO x adsorbent may be mixed with the at least one of a H 2 O adsorbent and a SC adsorbent.
  • At least one of H 2 O and HC in an exhaust gas is adsorbed on the at least one of a H 2 O adsorbent and a HC adsorbent, when the exhaust gas is passed through the plasma reactor.
  • the H 2 O adsorbed on the adsorbent is activated to be a strong oxidant such as OH radical and O radical
  • HC adsorbed on the adsorbent is activated to be radicals, lower HC, etc.
  • the obtained radicals facilitate the conversion from NO to NO 2 in the exhaust gas.
  • the resulting NO 2 tends to be more adsorbed onto the downstream NO x adsorbent than NO x . Therefore, according to the present apparatus, it is possible to prevent NO x from flowing out from an exhaust gas purifying apparatus at a lower temperature without increasing an amount of noble metal to be used.
  • the at least one of a H 2 O adsorbent and a HC adsorbent is selected from the group consisting of alumina, silica, zeolite, zirconia and meso-porous silica, and combinations thereof.
  • the NO x adsorbent is selected from the group consisting of zirconia and ceria, and a combination thereof.
  • the NO x adsorbent carries a NO x purifying catalyst.
  • adsorb NO 2 obtained by the at least one of a H 2 O adsorbent and a HC adsorbent onto the NO x adsorbent it is possible to adsorb NO 2 obtained by the at least one of a H 2 O adsorbent and a HC adsorbent onto the NO x adsorbent and to purify the adsorbed NO 2 thereon.
  • it comprises a NO x purifying catalyst located downstream of the NO x adsorbent.
  • the NO x desorbed from NO x adsorbent after the temperature of the NO x adsorbent is raised can be converted into N 2 by the NO x purifying catalyst which has already been warmed up.
  • the present apparatuses it is also possible to reduce an amount of noble metal, such as a platinum group metal, used in the NO x purifying catalyst, or further to omit the noble metal, because NO in the exhaust gas has been converted into NO 2 which is easily adsorbed onto a NO x adsorbent and a NO x purifying catalyst.
  • noble metal such as a platinum group metal
  • a method according to the present invention for using the present exhaust gas purifying apparatus comprises generating a plasma in the plasma reactor only when the temperature of the NO x purifying catalyst is lower than a predetermined temperature, e.g. lower than 300° C.
  • the present method it is possible to promote the conversion from NO to NO 2 , and thereby promote adsorption of NO x onto the NO x adsorbent, only when the NO x purifying catalyst is at the lower temperature such that NO x in an exhaust gas cannot be sufficiently purified by the NO x purifying catalyst. Further, according to the present method, it is possible to stop the generation of plasma, in order to save energy, when the NO x purifying catalyst is at a high enough temperature and has enough NO x purifying ability.
  • FIG. 1 is a block diagram of one embodiment of the present exhaust gas purifying apparatus.
  • FIG. 2 is a block diagram of another embodiment of the present exhaust gas purifying apparatus.
  • FIGS. 3 a and 3 b are respectively a perspective view and a cross sectional view of one plasma reactor which can be used for the present exhaust gas purifying apparatus.
  • FIGS. 4 a and 4 b are respectively a perspective view and a cross sectional view of another plasma reactor which can be used for the present exhaust gas purifying apparatus.
  • an exhaust gas emitted from an engine is passed through the plasma reactor containing at least one of a H 2 O adsorbent and a HC adsorbent, and then a NO x adsorbent located downstream of the plasma reactor.
  • an exhaust gas emitted from an engine is passed through a plasma reactor containing a NO x adsorbent, and at least one of a H 2 O adsorbent and a HC adsorbent in a plasma reactor.
  • the adsorbent in the plasma reactor adsorbs H 2 O and/or HC, and then the adsorbed H 2 O and/or HC is decomposed into radical chemicals by applying plasma.
  • the radial chemicals enhance oxidizing NO into NO 2 .
  • the H 2 O adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of a H 2 O adsorbent, e.g. at least one of alumina, silica and zeolite.
  • the HC adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of HC adsorbents, e.g. at least one of zeolite, zirconia and meso-porous silica.
  • the NO x adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of adsorbent which adsorbs NO x , in particular adsorbents which desorb the adsorbed NO x at the higher temperature, e.g. at least one of zirconia and ceria.
  • the adsorbents may be in any form, e.g. it can be formed into pellet form and charged into a container, or molded into a honeycomb structure. Further, it is also possible to coat the slurry containing the adsorbent onto a honeycomb structure substrate, and dry and fire the coated substrate.
  • the NO x purifying catalyst which is carried onto the NO x adsorbent and/or located downstream of the NO x adsorbent is any kind of catalyst a which catalyze reduction from NO x to N 2 .
  • the NO x purifying catalyst may be so-called a NO x storage reduction catalyst, a NO x selective reduction catalyst or a three-way catalyst.
  • the NO x storage reduction catalyst is a catalyst which stores NO x when air/fuel ratio is in lean, and reduces the stored NO x into N 2 when air/fuel ratio is in rich, i.e. when a rich spike is provided (when fuel is injected into an exhaust gas flow).
  • the NO x storage reduction catalyst is a catalyst wherein a porous metal oxide carrying one or more noble metal such as Pt, Rh, Pd, Ir and RU, and one or more NO x storage elements such as alkali metals, alkali earth metals and rare earth metals.
  • the NO x selective reduction catalyst is a catalyst which selectively reduce or decompose NO x in an exhaust gas.
  • the NO x selective reduction catalyst is used for purifying NO x in the exhaust gas emitted from an internal combustion engine driven at a lean air/fuel ratio.
  • the NO x selective reduction catalyst is a zeolite exchanged with transition metal such as Cu, or zeolite or alumina carrying one or more noble metal.
  • the three-way catalyst is a catalyst which converts CO, HC and NO x in an exhaust gas emitted from an internal combustion engine driven at near theoretical air/fuel ratio into CO 2 , H 2 O and N 2 .
  • the three way catalyst is an alumina carrying mixture of Pt and Rh, or mixture of Pd and Rh.
  • the catalyst metals such as noble metal can be carried onto any kind of substrate such as formed substrate or powder substrate by wash coating etc. Further, a formed substrate such as honeycomb substrate may be coated with the slurry comprising the particle carrying catalyst metals, and then dried and fired to obtain a formed substrate carrying catalyst.
  • the particle carrying catalyst metals may be formed into pallet form and charged into a container.
  • the plasma reactor used for the exhaust gas purifying apparatus of the present invention may be any kind of plasma reactor which allows the exhaust gas therethrough, e.g. the plasma reactor shown in FIGS. 3 a and 3 b .
  • FIG. 3 a shows a perspective view
  • FIG. 3 b shows a side sectional view of the plasma reactor.
  • the plasma reactor can generate plasma by a discharge between a center electrode 20 and outer electrode 30 , these electrodes 20 and 30 being connected to a power supply 40 .
  • the exhaust gas to be treated is passed through a cylindrical passage 10 surrounded by the outer electrode 30 .
  • the H 2 O and/or HC adsorbent, and optionally NO x adsorbent, may be positioned in the cylindrical passage 10 .
  • the center electrode 20 is made of a material that makes it possible to apply a voltage between the center electrode 20 and the outer electrode 30 .
  • the material may be electrically conductive material, electrically semi-conductive material etc. and especially, a metal such as Cu, W, stainless steel, Fe, PT and Al, the stainless steel being most preferable due to its durability and low cost.
  • the center electrode 20 may be a metal wire, but it may be a hollow bar.
  • the outer electrode 30 is made of a material that makes it possible to apply a voltage between the center electrode 20 and the outer electrode 30 .
  • the material may be the material described for the center electrode 20 .
  • the outer electrode 30 may be made by surrounding a mesh or foil of these materials around the cylindrical passage 10 , or by applying a conductive paste on the circumference surface of the cylindrical passage 10 .
  • the electric power supply 40 may be one supplying a pulse or constant direct current (DC), or alternating current (AC) voltage.
  • a voltage applied between the center electrode 20 and the outer electrode 30 and the pulse period of the applied voltage may be 50 kv and 2000 Hz.
  • the center electrode 20 may be a cathode or anode.
  • the electric power supply 40 may apply a high voltage between the center electrode 20 and outer electrode 30 in order to generate discharge and then plasma therebetween.
  • the electric power supply 40 may apply a DC voltage, AC voltage, a voltage having a periodic waveform, etc. between the electrodes.
  • DC pulse voltage is applied since it can generate a stable corona electric discharge.
  • the applied voltage, pulse width and pulse period of the DC pulse voltage may be optionally determined as long as it generates a corona electric discharge.
  • the applied voltage and pulse period are respectively a high voltage and short period in order to generate a corona electric discharge, though those parameters may be restricted by the design of the apparatus, an economical interest, etc.
  • the plasma reactor used for the exhaust gas purifying apparatus of the present invention may be the plasma reactor shown in FIGS. 4 a and 4 b .
  • FIG. 4 a shows a perspective view
  • FIG. 4 b shows a side sectional view of the plasma reactor.
  • the plasma reactor can generate plasma by discharging between mesh-like electrodes 25 and 35 which are upstream and downstream of the cylindrical passage 10 , these electrodes 25 and 35 being connected to a power supply 40 .
  • the exhaust gas to be treated is passed through an upstream mesh-like electrode 25 , cylindrical passage 10 and then a downstream-like mesh electrode 35 .
  • the H 2 O and/or HC adsorbent, and optionally NO x adsorbent, may be contained in the cylindrical passage 10 .
  • the description of the materials used for electrodes, the power supply, etc. of the plasma reactor of FIGS. 3 a and 3 b may be referred for the plasma reactor of FIGS. 4 a and 4 b.
  • a plasma reactor shown in FIGS. 3 a and 3 b was used to determine the effect of a H 2 O adsorbent and plasma on a conversion from NO to NO 2 .
  • silica pellets were contained in the plasma reactor as a H 2 O adsorbent.
  • the plasma reactor was located in the reaction tube of an laboratory model-gas analyzer, and the adsorbent temperature was maintained at the temperature of 150° C.
  • a gas comprising 1000 ppm of NO, 10% of O 2 , 3.0% of H 2 O and balance N 2 was passed through the plasma reactor to evaluate a conversion ratio from NO to NO 2 .
  • DC pulse discharge 50 kV, 2000 Hz
  • the example 1 was repeated except that the plasma was not generated.
  • the example 1 was repeated except that the H 2 O adsorbent was not contained in the plasma reactor.
  • the example 1 was repeated except that the plasma was not generated and the H 2 O adsorbent was not contained in the plasma reactor.
  • the example 1 according to the present invention (plasma+H 2 adsorbent) provides a much better performance than comparative example 1 (H 2 O adsorbent only), comparative example 2 (plasma only) and comparative example 3 (without plasma and B 2 O adsorbent).
  • FIGS. 3 a and 3 b A plasma reactor shown in FIGS. 3 a and 3 b was used to determine the effect of a HC adsorbent and plasma on a conversion from NO to NO 2 .
  • zeolite (ZSM-5) pellets was contained in the plasma reactor as a HC adsorbent.
  • the plasma reactor was located in the reaction tube of an laboratory model-gas analyzer, and the adsorbent temperature was maintained at the temperature of 150° C.
  • a gas comprising 1000 ppm of NO, 300 ppm of C 3 H 6 , 10% of O 2 , 3.0% of H 2 O and balance N 2 was passed through the plasma reactor to evaluate a conversion ratio from NO to NO 2 .
  • DC pulse discharge 50 kV, 2000 Hz
  • the example 2 was repeated except that the plasma was not generated.
  • the example 2 was repeated except that the HC adsorbent was not contained in the plasma reactor.
  • the example 2 was repeated except that the plasma was not generated and the HC adsorbent was not contained in the plasma reactor.
  • the example 1 according to the present invention provides a much better performance than comparative example 4 (HC adsorbent only), comparative, example 5 (plasma only) and comparative example 6 (without plasma and HC adsorbent).

Abstract

The present invention relates to an exhaust gas purifying apparatus comprising a plasma reactor containing at least one of a H2O adsorbent and a HC adsorbent; and a NOx adsorbent located downstream of said plasma reactor. Further, the present invention relates to an exhaust gas purifying apparatus comprising a plasma reactor containing at least one of a H2O adsorbent and a HC adsorbent, and a NOx adsorbent. Still further, the present invention relates to a method using the present apparatus. According to the present apparatus, by generating plasma in the plasma reactor, H2O trapped on the H2O adsorbent is activated to be a strong oxidant such as OH radical and O radical, and HC trapped on the HC adsorbent is activated to be radicals, lower HCs, etc.

Description

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for purifying exhaust gas emitted from internal combustion engine and, in particular, to an apparatus for purifying NOx (nitrogen oxide) emitted from a motor vehicle.
2. Description of Related Art
In order to purify an exhaust gas emitted from an internal combustion engine such as a motor vehicle, it is usual to reduce NOx in the exhaust gas by the use of NOx purifying catalyst carrying a noble metal. However, it is necessary to use a larger amount of noble metal, to sufficiently purifying NOx at a lower temperature, as the NOx purifying catalyst generally does not have much NOx purifying ability at the lower temperature. Using the larger amount of noble metal increases costs and does not always provides a satisfactory result.
Japanese Unexamined Patent Publication NO. 2001-182525 describes an exhaust gas purifying apparatus wherein plasma is generated in a plasma reactor to enhance the adsorption of NOx by a NOx adsorbent therein, when a NOx purifying catalyst located downstream thereof is at a low temperature and does not have enough NOx purifying ability. Once the catalyst in warmed up, the trapped NOx can be desorbed and purified at the downstream NOx purifying catalyst by stopping plasma.
The prior art has partially overcome the problem that the NOx purifying catalyst does not have enough NO, purifying ability before it is warmed up. However, the further improvements are required because the NOx purifying ability is not always enough even if the prior art is employed.
Therefore, the present invention provides an improved exhaust gas purifying apparatus, which overcome the above problems, and a method for using it.
BRIEF SUMMARY OF THE INVENTION
An exhaust gas purifying apparatus according to the present invention comprises a plasma reactor containing at least one of a H2O adsorbent and a HC (hydrocarbon) adsorbent; and a NOx adsorbent located downstream of said plasma reactor.
Another exhaust gas purifying apparatus according to the present invention comprises a plasma reactor containing at least one of a H2O adsorbent and a HC adsorbent, and a NOx adsorbent. The NOx adsorbent in the reactor may be positioned downstream of the at least one of a H2O adsorbent and a HC adsorbent. Further, the NOx adsorbent may be mixed with the at least one of a H2O adsorbent and a SC adsorbent.
In the use of the present exhaust gas purifying apparatuses, at least one of H2O and HC in an exhaust gas is adsorbed on the at least one of a H2O adsorbent and a HC adsorbent, when the exhaust gas is passed through the plasma reactor. On generating plasma in the plasma reactor, the H2O adsorbed on the adsorbent is activated to be a strong oxidant such as OH radical and O radical, and HC adsorbed on the adsorbent is activated to be radicals, lower HC, etc. The obtained radicals facilitate the conversion from NO to NO2 in the exhaust gas. The resulting NO2 tends to be more adsorbed onto the downstream NOx adsorbent than NOx. Therefore, according to the present apparatus, it is possible to prevent NOx from flowing out from an exhaust gas purifying apparatus at a lower temperature without increasing an amount of noble metal to be used.
In one embodiment of the present apparatus, the at least one of a H2O adsorbent and a HC adsorbent is selected from the group consisting of alumina, silica, zeolite, zirconia and meso-porous silica, and combinations thereof.
In another embodiment of the present apparatus, the NOx adsorbent is selected from the group consisting of zirconia and ceria, and a combination thereof.
In another embodiment of the present apparatus, the NOx adsorbent carries a NOx purifying catalyst.
According to this embodiment of the present apparatus, it is possible to adsorb NO2 obtained by the at least one of a H2O adsorbent and a HC adsorbent onto the NOx adsorbent and to purify the adsorbed NO2 thereon.
In another embodiment of the present apparatus, it comprises a NOx purifying catalyst located downstream of the NOx adsorbent.
According to this embodiment of the present apparatus, the NOx desorbed from NOx adsorbent after the temperature of the NOx adsorbent is raised can be converted into N2 by the NOx purifying catalyst which has already been warmed up.
In the present apparatuses, it is also possible to reduce an amount of noble metal, such as a platinum group metal, used in the NOx purifying catalyst, or further to omit the noble metal, because NO in the exhaust gas has been converted into NO2 which is easily adsorbed onto a NOx adsorbent and a NOx purifying catalyst.
A method according to the present invention for using the present exhaust gas purifying apparatus comprises generating a plasma in the plasma reactor only when the temperature of the NOx purifying catalyst is lower than a predetermined temperature, e.g. lower than 300° C.
According to the present method, it is possible to promote the conversion from NO to NO2, and thereby promote adsorption of NOx onto the NOx adsorbent, only when the NOx purifying catalyst is at the lower temperature such that NOx in an exhaust gas cannot be sufficiently purified by the NOx purifying catalyst. Further, according to the present method, it is possible to stop the generation of plasma, in order to save energy, when the NOx purifying catalyst is at a high enough temperature and has enough NOx purifying ability.
These and other objects, features and advantages of the present invention will become apparent to a person with ordinary skill in the art upon reading the following detailed description along with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of one embodiment of the present exhaust gas purifying apparatus.
FIG. 2 is a block diagram of another embodiment of the present exhaust gas purifying apparatus.
FIGS. 3 a and 3 b are respectively a perspective view and a cross sectional view of one plasma reactor which can be used for the present exhaust gas purifying apparatus.
FIGS. 4 a and 4 b are respectively a perspective view and a cross sectional view of another plasma reactor which can be used for the present exhaust gas purifying apparatus.
 DETAILED DESCRIPTION OF THE INVENTION
The present invention is described regarding embodiments and drawings which are not intended to limit the scope of the present invention shown in the claims.
The first and second embodiments of the present exhaust gas purifying apparatuses are described in relation to FIGS. 1 and 2, below.
In the use of the apparatus shown in FIG. 1, an exhaust gas emitted from an engine is passed through the plasma reactor containing at least one of a H2O adsorbent and a HC adsorbent, and then a NOx adsorbent located downstream of the plasma reactor. In the use of the apparatus shown in FIG. 2, an exhaust gas emitted from an engine is passed through a plasma reactor containing a NOx adsorbent, and at least one of a H2O adsorbent and a HC adsorbent in a plasma reactor. The adsorbent in the plasma reactor adsorbs H2O and/or HC, and then the adsorbed H2O and/or HC is decomposed into radical chemicals by applying plasma. The radial chemicals enhance oxidizing NO into NO2.
The components of the exhaust gas purifying apparatus shown in FIGS. 1 and 2 are described below in more detail.
The H2O adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of a H2O adsorbent, e.g. at least one of alumina, silica and zeolite. The HC adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of HC adsorbents, e.g. at least one of zeolite, zirconia and meso-porous silica.
The NOx adsorbent used for the exhaust gas purifying apparatus of the present invention may be any kind of adsorbent which adsorbs NOx, in particular adsorbents which desorb the adsorbed NOx at the higher temperature, e.g. at least one of zirconia and ceria.
The adsorbents may be in any form, e.g. it can be formed into pellet form and charged into a container, or molded into a honeycomb structure. Further, it is also possible to coat the slurry containing the adsorbent onto a honeycomb structure substrate, and dry and fire the coated substrate.
The NOx purifying catalyst which is carried onto the NOx adsorbent and/or located downstream of the NOx adsorbent is any kind of catalyst a which catalyze reduction from NOx to N2. The NOx purifying catalyst may be so-called a NOx storage reduction catalyst, a NOx selective reduction catalyst or a three-way catalyst.
The NOx storage reduction catalyst is a catalyst which stores NOx when air/fuel ratio is in lean, and reduces the stored NOx into N2 when air/fuel ratio is in rich, i.e. when a rich spike is provided (when fuel is injected into an exhaust gas flow). For example, the NOx storage reduction catalyst is a catalyst wherein a porous metal oxide carrying one or more noble metal such as Pt, Rh, Pd, Ir and RU, and one or more NOx storage elements such as alkali metals, alkali earth metals and rare earth metals.
The NOx selective reduction catalyst is a catalyst which selectively reduce or decompose NOx in an exhaust gas. The NOx selective reduction catalyst is used for purifying NOx in the exhaust gas emitted from an internal combustion engine driven at a lean air/fuel ratio. For example, the NOx selective reduction catalyst is a zeolite exchanged with transition metal such as Cu, or zeolite or alumina carrying one or more noble metal.
The three-way catalyst is a catalyst which converts CO, HC and NOx in an exhaust gas emitted from an internal combustion engine driven at near theoretical air/fuel ratio into CO2, H2O and N2. For example, the three way catalyst is an alumina carrying mixture of Pt and Rh, or mixture of Pd and Rh.
The catalyst metals such as noble metal can be carried onto any kind of substrate such as formed substrate or powder substrate by wash coating etc. Further, a formed substrate such as honeycomb substrate may be coated with the slurry comprising the particle carrying catalyst metals, and then dried and fired to obtain a formed substrate carrying catalyst. The particle carrying catalyst metals may be formed into pallet form and charged into a container.
The plasma reactor used for the exhaust gas purifying apparatus of the present invention may be any kind of plasma reactor which allows the exhaust gas therethrough, e.g. the plasma reactor shown in FIGS. 3 a and 3 b. FIG. 3 a shows a perspective view and FIG. 3 b shows a side sectional view of the plasma reactor.
The plasma reactor can generate plasma by a discharge between a center electrode 20 and outer electrode 30, these electrodes 20 and 30 being connected to a power supply 40. The exhaust gas to be treated is passed through a cylindrical passage 10 surrounded by the outer electrode 30. The H2O and/or HC adsorbent, and optionally NOx adsorbent, may be positioned in the cylindrical passage 10.
The center electrode 20 is made of a material that makes it possible to apply a voltage between the center electrode 20 and the outer electrode 30. The material may be electrically conductive material, electrically semi-conductive material etc. and especially, a metal such as Cu, W, stainless steel, Fe, PT and Al, the stainless steel being most preferable due to its durability and low cost. The center electrode 20 may be a metal wire, but it may be a hollow bar.
The outer electrode 30 is made of a material that makes it possible to apply a voltage between the center electrode 20 and the outer electrode 30. The material may be the material described for the center electrode 20. The outer electrode 30 may be made by surrounding a mesh or foil of these materials around the cylindrical passage 10, or by applying a conductive paste on the circumference surface of the cylindrical passage 10.
The electric power supply 40 may be one supplying a pulse or constant direct current (DC), or alternating current (AC) voltage. A voltage applied between the center electrode 20 and the outer electrode 30 and the pulse period of the applied voltage may be 50 kv and 2000 Hz. The center electrode 20 may be a cathode or anode.
The electric power supply 40 may apply a high voltage between the center electrode 20 and outer electrode 30 in order to generate discharge and then plasma therebetween. The electric power supply 40 may apply a DC voltage, AC voltage, a voltage having a periodic waveform, etc. between the electrodes. Preferably, DC pulse voltage is applied since it can generate a stable corona electric discharge. The applied voltage, pulse width and pulse period of the DC pulse voltage may be optionally determined as long as it generates a corona electric discharge. Preferably, the applied voltage and pulse period are respectively a high voltage and short period in order to generate a corona electric discharge, though those parameters may be restricted by the design of the apparatus, an economical interest, etc.
The plasma reactor used for the exhaust gas purifying apparatus of the present invention may be the plasma reactor shown in FIGS. 4 a and 4 b. FIG. 4 a shows a perspective view and FIG. 4 b shows a side sectional view of the plasma reactor.
The plasma reactor can generate plasma by discharging between mesh- like electrodes 25 and 35 which are upstream and downstream of the cylindrical passage 10, these electrodes 25 and 35 being connected to a power supply 40. The exhaust gas to be treated is passed through an upstream mesh-like electrode 25, cylindrical passage 10 and then a downstream-like mesh electrode 35. The H2O and/or HC adsorbent, and optionally NOx adsorbent, may be contained in the cylindrical passage 10. The description of the materials used for electrodes, the power supply, etc. of the plasma reactor of FIGS. 3 a and 3 b may be referred for the plasma reactor of FIGS. 4 a and 4 b.
The effects of the present invention are shown with regard to the examples, which are not intended to limit the scope of the present invention shown in the claims.
EXAMPLE 1
A plasma reactor shown in FIGS. 3 a and 3 b was used to determine the effect of a H2O adsorbent and plasma on a conversion from NO to NO2.
In this example, 2.0 g of silica pellets was contained in the plasma reactor as a H2O adsorbent. The plasma reactor was located in the reaction tube of an laboratory model-gas analyzer, and the adsorbent temperature was maintained at the temperature of 150° C. A gas comprising 1000 ppm of NO, 10% of O2, 3.0% of H2O and balance N2 was passed through the plasma reactor to evaluate a conversion ratio from NO to NO2. DC pulse discharge (50 kV, 2000 Hz) was used to generate plasma.
COMPARATIVE EXAMPLE 1
The example 1 was repeated except that the plasma was not generated.
COMPARATIVE EXAMPLE 2
The example 1 was repeated except that the H2O adsorbent was not contained in the plasma reactor.
COMPARATIVE EXAMPLE 3
The example 1 was repeated except that the plasma was not generated and the H2O adsorbent was not contained in the plasma reactor.
The results obtained in example 1 and comparative examples 1 to 3 are shown in the following Table 1.
TABLE 1
NO → NO2
conversion ratio
no plasma plasma
no H2O adsorbent 0 18
H2O adsorbent 1 40
As seen from Table 1, the example 1 according to the present invention (plasma+H2 adsorbent) provides a much better performance than comparative example 1 (H2O adsorbent only), comparative example 2 (plasma only) and comparative example 3 (without plasma and B2O adsorbent).
EXAMPLE 2
A plasma reactor shown in FIGS. 3 a and 3 b was used to determine the effect of a HC adsorbent and plasma on a conversion from NO to NO2.
In this example, 2.0 g of zeolite (ZSM-5) pellets was contained in the plasma reactor as a HC adsorbent. The plasma reactor was located in the reaction tube of an laboratory model-gas analyzer, and the adsorbent temperature was maintained at the temperature of 150° C. A gas comprising 1000 ppm of NO, 300 ppm of C3H6, 10% of O2, 3.0% of H2O and balance N2 was passed through the plasma reactor to evaluate a conversion ratio from NO to NO2. DC pulse discharge (50 kV, 2000 Hz) was used to generate plasma.
COMPARATIVE EXAMPLE 4
The example 2 was repeated except that the plasma was not generated.
COMPARATIVE EXAMPLE 5
The example 2 was repeated except that the HC adsorbent was not contained in the plasma reactor.
COMPARATIVE EXAMPLE 6
The example 2 was repeated except that the plasma was not generated and the HC adsorbent was not contained in the plasma reactor.
The results obtained in example 2 and comparative examples 4 to 6 are shown in the following Table 2.
TABLE 2
NO → NO2
conversion ratio
no plasma plasma
no HC adsorbent 0 29
HC adsorbent 10 71
As seen from Table 2, the example 1 according to the present invention (plasma+HC adsorbent) provides a much better performance than comparative example 4 (HC adsorbent only), comparative, example 5 (plasma only) and comparative example 6 (without plasma and HC adsorbent).
Although the present invention has been fully described by way of the example with reference to the accompanying drawings, it should be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, such changes and modifications can be made within the scope of the present invention hereinafter defined.

Claims (8)

1. An exhaust gas purifying apparatus, comprising:
a plasma reactor containing at least one of a H2O adsorbent and a HC adsorbent, and
a NOx adsorbent located downstream of said plasma reactor, wherein:
said NOx adsorbent has a NOx purifying catalyst, or the exhaust gas purifying apparatus has the NOx purifying catalyst located downstream of the NOx adsorbent, and
plasma is generated in the plasma reactor only when a temperature of said NOx purifying catalyst is lower than a predetermined temperature to facilitate the conversion of NO to NO2 in the exhaust gas.
2. The exhaust gas purifying apparatus according to claim 1, wherein said predetermined temperature is 300° C.
3. The exhaust gas purifying apparatus according to claim 1, wherein said at least one of the H2O adsorbent and the HC adsorbent is selected from the group consisting of alumina, silica, zeolite, zirconia and meso-porous silica, and combinations thereof.
4. The exhaust gas purifying apparatus according to claim 1, wherein said NOx adsorbent is selected from the group consisting of zirconia and ceria, and a combination thereof.
5. An exhaust gas purifying apparatus, comprising:
a plasma reactor containing at least one of a H2O adsorbent and a HC adsorbent, and a NOx adsorbent, wherein:
said NOx adsorbent has a NOx purifying catalyst, or the exhaust gas purifying apparatus has the NOx purifying catalyst located downstream of the NOx adsorbent, and
plasma is generated in the plasma reactor only when a temperature of said NOx purifying catalyst is lower than a predetermined temperature to facilitate the conversion of NO to NO2O in the exhaust gas.
6. The exhaust gas purifying apparatus according to claim 5, wherein said predetermined temperature is 300° C.
7. The exhaust gas purifying apparatus according to claim 5, wherein said at least one of the H2O adsorbent and the HC adsorbent is selected from the group consisting of alumina, silica, zeolite, zirconia and meso-porous silica, and combinations thereof.
8. The exhaust gas purifying apparatus according to claim 5, wherein said NOx adsorbent is selected from the group consisting of zirconia and ceria, and a combination thereof.
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